1. Field of the Invention
The present invention relates generally to light emitting diode (LED) based lighting apparatuses, and more particularly to an apparatus for driving an LED based lighting apparatus having multi-color LED strings.
2. Description of Related Arts
LEDs are semiconductor-based light sources often employed in low-power instrumentation and appliance applications for indication purposes. The application of LEDs in various lighting units has become more and more popular. For example, high brightness LEDs have been widely used for traffic lights, vehicle indicating lights, and braking lights.
An LED has an I-V characteristic curve similar to an ordinary diode. When the voltage applied to the LED is less than a forward voltage, only very small current flows through the LED. When the voltage exceeds the forward voltage, the current increases sharply. The output luminous intensity of an LED light is approximately proportional to the LED current for most operating values of the LED current except for the high current value. A typical driving device for an LED light is designed to provide a constant current for stabilizing light emitted from the LED and extending the life of the LED.
In order to increase the brightness of an LED light, a number of LEDs are usually connected in series to form an LED-based lighting string and a number of LED-based lighting strings may further be connected in series to form a lighting apparatus. For example, U.S. Pat. No. 6,777,891 discloses a plurality of LED-based lighting strings as a computer-controllable light string with each lighting string forming an individually-controllable node of the light string.
The operating voltage required by each lighting string typically is related to the forward voltage of the LEDs in each lighting string, how many LEDs are employed for each of the lighting string and how they are interconnected, and how the respective lighting strings are organized to receive power from a power source. Accordingly, in many applications, some type of voltage conversion device is required in order to provide a generally lower operating voltage to one or more LED-based lighting strings from more commonly available higher power supply voltages. The need of a voltage conversion device reduces the efficiency, costs more and also makes it difficult to miniaturize an LED-based lighting device.
U.S. Pat. No. 7,781,979 provides an apparatus for controlling series-connected LEDs. Two or more LEDs are connected in series. A series current flows through the LEDs when an operating voltage is applied. One or more controllable current paths are connected in parallel with at least an LED for partially diverting the series current around the LED. The apparatus permits the use of operating voltages such as 120V AC or 240V AC without requiring a voltage conversion device.
US Pat. Publication No. 2010/0308739 discloses a plurality of LEDs coupled in series to form a plurality of segments of LEDs and a plurality of switches coupled to the plurality of segments of LEDs to switch a selected segment into or out of a series LED current path in response to a control signal. US Pat. Publication No. 2011/0085619 discloses an LED selection circuit for an LED driver that drives multiple unequal lengths of LED strings to selectively turn the LED strings on and off corresponding to an input AC line voltage. US Pat. Publication No. 2012/0217887 discloses LED lighting systems and control methods capable of providing an average luminance intensity independent from the variation of an AC voltage.
As more and more LED-based lighting strings are used in high brightness lighting equipment, there is a strong need to design methods and apparatus that can drive and connect the LED-based lighting strings intelligently and efficiently to increase the utilization of the LEDs and provide stable and high brightness by using the readily available AC source from a wall power unit.
In principle, it is possible to generate a light of any desirable color if LEDs of red, green and blue colors are assembled together in a lighting apparatus. In order to operate under the readily available AC voltage, a multi-color LED lighting apparatus presents a further challenge in the design of its driving circuit because the number of LEDs in each color and how the LEDs of different colors are connected in series or parallel have to be considered in addition to the variation of the input AC voltage.
There is a strong need in providing an efficient and flexible driving circuit for the multi-color LED lighting apparatus to generate lights of different colors and different brightness under different lighting and color requirements.